In the production of lime from limestone, the limestone is heated in a calcining chamber so as to decompose the same and drive off carbon dioxide according to the general equation: EQU CaCO.sub.3 +Heat.fwdarw.CaO+CO.sub.2
with heating of the limestone to a high temperature and holding it at that temperature for a period of time to effect the decomposition. This calcination, or "burning", requires a large amount of heat.
When calcining of limestone is effected in a rotary kiln, the kiln generally is a long, straight tubular steel vessel that is lined on the inside with refractory material. The kiln rotates about an axis at an angle of a few degrees, such as 3.degree. to 5.degree., to the horizontal, and rests on trunnions which rotate the tubular shell. Limestone, calcium carbonate, is introduced to a calcining chamber in the rotary vessel at the upper end and is decomposed, by heat, to calcium oxide which is extracted at the lower end. A combustor is attached at the lower end and burning fuel, such as oil, natural gas, or coal provides heat to the lower end of the calcining chamber of the rotary vessel. Combustion products and carbon dioxide are removed from the upper end of the rotary vessel. Little attempt is made to recover heat other than quenching the hot calcium oxide in the air stream which is, in turn, used in the combustion process. The exhaust gases from the kiln may be cleaned by use of a wet scrubbing unit.
In recent years, rotary lime kilns have operated using natural gas or oil as fuel in the combustor, but coal is a preferred fuel due to its lower cost. A typical small kiln with an 81/2 feet diameter casing, 150 feet long, and an inner diameter of 71/2 feet, has an output of about 200 tons per day of lime and uses 0.275 tons of coal per ton of lime produced. This represents a heat input of 60.times.10.sup.6 BTU/hour when burning a typical coal containing 6.9 percent ash, 2 to 3 percent sulfur and having a higher heating value of 13,000 BTU/pound.
The use of coal as a fuel in firing a lime kiln results in molten slag, in dispersed form, being fed to the calcining chamber along with the hot combustion gases produced. This molten slag, when contacting the cooler inner surface of the rotary kiln adheres to the surface and subsequently forms a slag ring. After a period of operation, the slag ring builds up to a size such that a major blockage to flow through the kiln results, and the kiln must be shut down and the slag ring physically removed. The removal of the slag ring requires many mandays of effort, with this maintenance expense incurred in addition to the loss of production of the kiln.
It is an object of the present invention to provide a method for coal firing of a lime kiln which retards the build-up of a slag ring in the calcining chamber of the kiln by removing the bulk of the molten slag, or ash, from the combustion gases prior to introduction of the gases into the calcining chamber.
It is another object of the present invention to provide an improved lime kiln for use with coal combustion wherein a two stage combustor is used, with at least 75 percent of molten slag formed removed from the first stage of the two stage combustor, prior to introduction of hot combustion gases into the calcining chamber of a rotary lime kiln.